Solid Waste Processing with Hydrogen Peroxide
Basis of Control
H2O2
controls odors and corrosion within gravity thickeners by direct
oxidation of H2S (as it rises from the solids blanket) and, to a lesser
degree, by prevention of odor generation (by supplying dissolved
oxygen). Control is typically achieved by adding 10-20 mg/L H2O2 to the
influent to the thickeners. Higher doses or alternate modes of addition
may be required in cases where: 1) hydraulic retention times are >
2-3 hours; 2) solids blanket depths are > 2-4 feet; or 3) waste
activated sludge is co-thickened with the primary solids. It is
generally not practical to eliminate odors within the thickened sludge -
supplemental post-treatment will be needed if this is important (see
below).
For
solids dewatering (e.g., by centrifuge or belt filter press), the
mechanism of control is direct oxidation of H2S within the feed solids.
The range of suitable reaction times is 2-30 minutes prior to the
dewatering device. The efficiency of treatment depends upon the
available reaction time, the initial level of H2S, and the relevance of
non-H2S odors. Under optimal conditions, effective dose ratios are about
5 parts H2O2 per part aqueous sulfide, and can be reliably estimated
through beaker tests.
Practical Considerations
The
most frequent obstacle to implementing H2O2 for solids dewatering
involves providing sufficient reaction time. Although a minimum of 2-3
minutes is recommended, there are certain instances (where iron levels
are > 10-20 mg/L) where 60-70% completion is achieved within one
minute.
In some cases, a secondary issue may arise with regard to
non-H2S (organic) odors. In such cases, longer reaction times should be
considered. This will allow the slower reaction of H2O2 with these
organics to proceed, while encouraging the bio-mediated oxidation of
these odors through supply of dissolved oxygen.
The effect of
this treatment on thickening/dewatering efficiencies is neutral (if H2O2
is replacing an existing chemical treatment) to positive (if no prior
sulfide treatment exists). At the doses used in these applications, H2O2
will not interfere with the performance of most polymers.
The
availability of supplemental odor treatment (e.g., odor scrubbers) will
affect the economics of treatment in two ways. First, it will lessen the
need to control sulfides to very low levels; and second, significant
credits will be afforded by the reduced operating cost (chemical use)
within the supplemental treatment device.
Evaluation Process
The
evaluation process for odor control in solids dewatering is
straightforward and reasonably accurate estimates can be made before
progressing to a field study. The preliminary assessment involves two
aspects: a review of facilities design and plant operating records; and
an on-site beaker study of the H2O2 - sulfide reaction within the sludge
matrix (to determine the extent of removal at varying reaction times
and dose ratios). This may be followed by a field pilot test to confirm
the preliminary findings.
The evaluation process for gravity
thickeners can be more complicated but still, reasonable estimates can
be made by reviewing the facilities design and plant operating records;
and performing on-site beaker tests.
The following information will greatly assist in completing a preliminary assessment:
- Schematic
of unit processes showing: the number, location and sizing of
clarifiers, thickeners, holding tanks, and dewatering devices, including
the sizing and length of the interconnecting piping;
- Average daily flows to and from these units;
- Description of the dewatering system (number, type and capacity of devices);
- Operating
records showing: the amount and type of sludge processed; the
schedule/frequency of operation; and daily or seasonal variations on
these practices;
- If appropriate, estimated hydraulic retention times and solids blanket depths within the thickeners;
- Current
(and abandoned) chemical injection points, if any, and type of
chemical, dosing rates, costs, and any comments on effectiveness;
- Existence
of supplemental vapor treatment such as odor scrubbers, including
information regarding scope of coverage, gas flow rate,
influent/effluent H2S concentrations, and chemical use-rates and costs;
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